1. Field of the Invention
The present invention relates to a secondary surveillance radar system and a ground equipment for use therein. Also, the present invention relates to improvement of a method for determining validity/invalidity of reply signals returned from an airborne equipment in reply to interrogation signals transmitted from the ground equipment, the method being applied to the ground equipment. The airborne equipment is an equipment mounted in an aircraft, such as a mode S transponder or an ATCRBS (Air Traffic Control Radar Beacon System) transponder.
2. Description of the Related Art
A secondary surveillance radar (SSR) system comprises a ground equipment for transmitting interrogation signals to an aircraft and an airborne equipment for returning reply signals in reply to the interrogation signals. It should be noted that a related technique is disclosed on pages 227–233 of “Radar Engineering Revised Edition” written by Takashi Yoshida, and published by the Institute of Electronics, Information and Communication Engineers (1996). This literature discloses a secondary surveillance radar in detail.
Conventionally, an ATCRBS (Air Traffic Control Radar Beacon System) transponder is used as the airborne equipment. In addition, in recent years, the number of aircrafts including mode S transponders has been increased. A mode S transponder has a function of incorporating various information into reply signals. By virtue of this function, the mode S transponder can transmit a larger number of information to the ground equipment.
The method of transmitting interrogation signals is variously designed in order that the ground equipment could acquire both the ATCRBS transponder and the mode S transponder.
In the SSR system, the time period for which interrogation signals are transmitted is divided into all-call time periods and roll-call time periods in units of one sweep time period of a radar. The all-call time period is a time period for acquiring the mode S transponder and the ATCRBS transponder. The roll-call time period is a time period in which an individual interrogation is sent to the mode S transponder, and a reply to the interrogation is returned from the mode S transponder. In a control zone, a number of aircrafts can be acquired with respective antenna main beams. Therefore, it is important how the all-call time periods and the roll-call time periods are set.
In a conventional secondary surveillance radar system, an all-call interrogation specific for mode S (mode S only all call) and an all-call interrogation specific for mode A (mode A only all call)(or an all-call interrogation specific for mode C (mode C only all call)) are transmitted in the same all-call time period such that the time interval between the time when the all-call interrogation specific for mode S is transmitted and the time when the all-call interrogation specific for mode A (or all-call interrogation specific for mode C) is transmitted is fixed. In this case, the above fixed time interval corresponds to the time interval between a sync phase reversal during an all call specific for mode S and a pulse P3 during an all call specific for mode A (or an all call specific for mode C).
Thus, if side lobe suppression using a pulse P2 fails, that is, all-call interrogations specific for mode S cannot be suppressed, there is a case where reply signals which should not be returned are returned from the ATCRBS transponder. That is, the ATCRBS transponder recognizes the sync phase reversal of P6 pulse as a P3 pulse. They interfere with surveillance of the airspace. Thus, this problem is required to be solved.
In such a manner, in the conventional secondary surveillance radar system, there is a case where invalid responses to all calls specific for mode S are returned from the ATCRBS transponder. In this case, in the ground equipment, in one sweep time period, invalid responses and valid responses are mixed, or only invalid responses can be obtained. The invalid responses interfere with surveillance of the airspace. They thus need to be avoided by some measure.